Solar Shed

Chronicles of my efforts to install an automated solar powered air conditioner into our shed.

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Current configuration:

(7) 100W Mono panels

(2) 20 AMP PWM Charge controllers

(7) 20A inline MC4 Diodes

(7) 10A inline MC4 Fuses

(1) 20A MPPT Solar charge controller

(1) 50A DC-to-DC charge controller w/MPPT

(1) 30A Mini-ANL fuse

(1) 80A Mini-ANL fuse

(1) 70A Mini-ANL fuse

(1) 150A Mini-ANL fuse

(1) 60A Bolt-on fuse

(2) 80A Bolt-on fuse

(1) 250A Termination block

(1) 1500W Inverter – pure sine

(2) 12V Deep cycle marine battery

(1) LiFePO4 50Ah battery

(1) 410W Window mounted air-conditioner

(1) Set 30A MC4 branch adapter 1:4

(1) Set 8AWG MC4 cables 15ft

(4) Set 10AWG MC4 cables 15ft

(2) Mini-ANL terminal blocks

(1) 4″ DIN Rail

(1) 25A DC DP-DT Circuit Breaker

(1) 32A DC DP-DT Circuit Breaker

4 AWG cable (black and red, about 8ft)

6 AWG cable (black and red, about 8ft)

8 AWG cable (black and red, about 16ft)

(1) MC4 Inline power meter 8AWG

(1) Inline power meter with shunt and display

(1) 12VDC Switchbox

(1) 24VDC Switchbox

(1) 5-24VDC Timer Relay

(1) 24VDC cooling fans

(1) Triple 40A Circuit breaker

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Current Activity:

A Renogy DC-to-DC battery charger is the heart of my system, it was a perfect way for me to add lithium to my existing lead acid batteries I didnt want to throw out. However, I should not have assumed this unit would work with any run of the mill 100W panel (like pretty much any cheapo PWM or MPPT controller might) because it has a very low voltage ceiling and will not charge anything if solar input is >25V. The open circuit voltage on my particular brand of panels (TP-Solar) is 27V, and then anywhere from 19V to 26V when connected. If there are panels connected >25V, the unit will basically halt and neither battery will be charged until solar is disconnected momentarily to clear the over-voltage state (draining your batteries if there is a load on them).

I tried to work around this oversight by adding a 20A inline MC4 blocking diode to each panel (when you add a diode inline voltage is reduced by .7V), but I still intermittently found it in the state where it was not charging, so I added a higher current blocking diode (for 1.4V drop) just before the charge controller, and that reduced the frequency of the problem. The high current diode generated quite a bit of heat though, so I had it mounted to a heat-sink and was cooling it with the 24v’ish input power from solar with 24VDC fans, so they did not use any battery at all.

I suspect the whole problem could be addressed with firmware by Renogy. All the unit needs to do is keep polling to see if still >25 and take action when this changes, manual intervention to disconnect/reconnect solar to eliminate >25 state should NOT be required 🙁

Adding heat to my environment was not desirable anyway so I found a superior alternative: Not using any solar at all on the Renogy controller. I added two cheap PWM controllers that charge the lead acid bank in place of the Renogy’s solar, it will use ‘alternator’ side only to charge the lithium battery, with no potential for the over voltage hangup, no extra heat, and I regain at least 50W that was being wasted.

Finally the overall system is fully automated with alot less worry (still gotta keep an eye on the weather, as long as its not hot AND cloudy Im good, but to have no worries I really need more lithium so I can deal with a few days like that in a row without intervention).

Here is a rough sketch of how I am planning to reconfigure things even further once I get more circuit breakers, some battery switches, another timer, longer DIN rail, and maybe a Renogy panel that works with their controller <25V open circuit. This reconfiguration will allow room to grow a little, could add 4 more panels for a total of 11 covering every surface the sun hits directly. I should really probably shitcan the PWM controllers and just get a larger capacity MPPT, but I have 5 laying around due to bundle purchases so I might as well utilize them.

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More future plans:

To insulate, I am planning to first add caulking, and then reflective air bubble insulation over that. Might also need some stripping around door to make it seal better, I can see a little bit of light from outside when everything is closed up.

(1) 250W Heat lamp – Now that cooling is automated, looking into possibilities to automate heating in the winter.

(1) 1200W Thermostat – I plan to use something like this to automatically switch between my summer (cooling) and winter (heating) loads.

(1) 500F Supercapacitor – This could help with voltage drop when compressor kicks in (rough on the equipment). Supercaps are known to greatly improve battery life, and last a very long time compared to batteries. Unfortunately they are cost prohibitive. To get the size needed to use my inverter with a constant load of 1500 watts I’d want 200 Farads which is not cheap. May be premature to invest in one of these now, but I’m very eager to experiment with these. I really hate lead acid, when my lead acid batteries are all used up I could see putting one of these in their place, and/or maybe even try a small 32V module for panel side of charger(s). Since MPPT chargers are effectively buck converters, they could be useful for draining a capacitor’s stored energy from high to low voltage, tapping their potential in a different way, although the only benefit that comes to mind doing something like that would be stabilizing power for X minutes when a cloud passes.

(1) Low Voltage Disconnect – Draining a battery too far can ruin it, and batteries can be expensive

(1) 600W Grid-tie plugin inverter – I am often producing more solar than I am consuming, so I need to start throwing the excess at our electric bill. From what I understand these plug-in inverters are super easy, you just plug them into the wall to spin your meter backwards. Hard part is going to be figuring out the transfer mechanisms between what the off-grid needs, and what this thing gets. Grey as to whether this Amazon purchase would be legal to use, I suspect not. Whatever I end up buying must meet the following requirements based on my research thus far:

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